Generally, in engines installed in vehicles such as automobiles, a piston is coupled to a smaller end part of a connecting rod by a piston pin. Specifically, the piston pin is inserted through a pin insertion hole formed in the smaller end part of the connecting rod, and the smaller end part of the connecting rod is located in a central area of the piston pin along the central axis direction. Two boss parts are formed in a bottom face of the piston (the face opposite to a top face, which is the face on the opposite of the combustion chamber side), at positions corresponding to both end portions of the piston pin along the central axis direction so as to sandwich the smaller end part of the connecting rod therebetween. The two boss parts are formed with pin supporting holes into which both the end portions of the piston pin along the central axis direction are inserted and for supporting both the end portions, respectively (e.g., see JP2004-353500A).
Such engines having the above configuration are known to cause combustion noise due to resonance caused by a basic structure of the engine (e.g., see Masaya Otsuka, “How to Minimize Diesel Combustion Noise by Improving Engine Structure,” Proceedings of Society of Automotive Engineers Convention, No. 36-05, Society of Automotive Engineers of Japan, Inc. May 2005, P. 7-10). In “How to Minimize Diesel Combustion Noise by Improving Engine Structure,” it is described that the engine sound has three peaks at 1.7 kHz, 3.3 kHz, and 6 kHz. One of these peaks (3.3 kHz) is caused by stretching resonance of the connecting rod, and the amplitude of this resonance is difficult to reduce.